Instead of connecting the MD21 HL microphone directly to a 400mH input transformer which make it sound bass heavy for voice an intermediate, in between, transformer was connected.
Background: When the MD21 is connected in parallel with an inductance of approximately 100mH, flat perceived frequency responce to listener's brain results when especially when used for softly speaking or singing voice. The rather small inductance reduces bass to compensate for Fletcher Munson and voice effort curves.
So the MD21 HL at L, was connected to a BBC transformer of input inductance 102mH and secondary inductance 336mH. This transformer step up ratio is a bit less than 1:2. The added advantage is the increase in level of the mic and the impedance is raised from 200Ω to a bit more than 600Ω.
The following transformer is the standard Canford in line which has an input inductance of 400mH. Secondary inductance is 95H. So the turns ratio is about 1:15.
Therefore the overall ratio becomes aproximately 1:30 and the MD21 impedance becomes now 200Ωx30x30=180KΩ
This may not be optimum for the Pleiades V4 grid as it has an input impedance of aproximately 100KΩ. It worked nicely.
Setup, signal path:
MD21 HL at L - 102mH primary (BBC LL/210A) secondary 336mH - 400mH Canford 95H - Pleiades V4, Vb=3.4V - mic in of Realistic 32-1101A Disco Mixer - HD580
Very full sound, very loud, correct bass balance to listener's brain when male singing and speaking at low levels close to the mic.
The Beyer M55 connected directly to Canford as it does not need bass compensation for this application still sounded more natural when used at 1in from mouth. Level was of course less so the fader was pushed a bit further up. It sounded surprisingly quiet too with the Canford and Pleiades V4 combination. It also sounded less bright making the sound more creamy smooth and natural. This compensation again seems to bring flat frequency responce to listener's brain due to Fletcher Munson and voice effort curves.
Setup, signal path:
Beyer M 55 - 400mH Canford 95H - Pleiades V4 - Realistic 32-1101A - HD580
There was some noise picked up possibly from the unsealed transformers on the MD21 setup.
A future experiment may be a Pleiades transformer with a Magnetec Nanoperm core doing the job of stepping up from 100mH to 50H at one go. So bass will be compensated and the 200Ω impedance of the MD21 stepped up to 100KΩ to match the grid of the EF183 electron tube of the Pleiades V4 battery electron tube preamplifier.
Even more high frequency detail is expended, not nessasarily a good thing. Perhaps having a higher stepup ratio and mismatching to grid is better but how far? May be needed to be found by experiment.
On these experiments the MD21 sounded, as expected, better at 45-90 degrees incidence to compensate for the pressure doubling effect that increases high frequencies on pressure microphones.
The Beyer M55 at 0 degrees. Is this too taken to account in this great microphone?
How would the MD21 sound if connected to SONY TCD5 Pro now being 600Ω stepped up trough the BBC transformer? The signal to noise ratio should now be superb as there would not be need to increase the Sony preamplifier gain too far near maximum.
This post has the inductance matching title as the secondary inductance of a transformer of 336mH is connected to the primary inductance of 400mH of the following transformer.
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